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1.
BECKER  HARRY 《Journal of Petrology》1996,37(4):785-810
Gamet-bearing high-temperature peridotite massifs in lower Austriawere exhumed during Carboniferous plate convergence in the Bohemianmassif. The peridotite massifs contain garnet pyroxenite layers,most of which are high-pressure cumulates that crystallizedin the deep lithosphere during ascent and cooling of hot asthenosphericmelts. Many of the pyroxenites have negative Eu anomalies andhigh LREE abundances in pyroxenes and bulk rocks, 87Sr/86Sr(335 Ma) as high as 0.7089, and Nd (335 Ma) as low as –4.8(leached clinopyroxenes and garnets). These pyroxenites alsoshow strong depletions in Rb, K, Ta, P and Ti compared withthe REE Equilibrium melt compositions calculated from the cumulatecompositions have very high LREE abundances (Lan = 300–600)and show strong LREEfractionation [(La/Sm)n = 7–47)].Trace element abundances, the Ca–Al-rich composition ofthe cumulates and possible Ti saturation in the melts suggestthat these melts were of primitive carbonatitic–meliliticor lamprophyrt-like composition. Other garnet pyroxenites suchas Al-rich garnet-kyanite clinopyroxemtes with positive Eu anomaliesprobably represent metamorphosed crustal rocks which were subductedand accreted to the lithospheric mantle. The high 87Sr/86Sr,low Nd (335 Ma) and negative Eu anomalies of the high-pressurecumulates can be explained if their equilibrium melts containeda component derived from subducted upper-crustal rocks. Thehigh equilibration pressures of the host peridotites (3–3.5GPa) and the high equilibration temperatures of the pyroxenites(1100–1400C) indicate that these melts are likely tobe derived from the sub-lithospheric mantle. There, meltingmay have been triggered by small amounts of melt or fluids derivedfrom a subducting slab at greater depth. KEY WORDS: garnet pyroxenites; geochemistry; lower Austria; ultramafic massifs; subduction  相似文献   

2.
The Ronda orogenic peridotite (southern Spain) contains a varietyof pyroxene-rich rocks ranging from high-pressure garnet granulitesand pyroxenites to low-pressure plagioclase–spinel websterites.The ‘asthenospherized’ part of the Ronda peridotitecontains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur asswarms of layers showing gradual modal transitions towards theirhost peridotites. Previous studies have suggested that theselayered pyroxenites formed by the replacement of refractoryspinel peridotites. Here, we present a major- and trace-element,and numerical modelling study of a layered outcrop of groupC pyroxenite near the locality of Tolox aimed at constrainingthe origin of these pyroxenites after host peridotites by pervasivepyroxene-producing, refertilization melt–rock reactions.Mg-number [= Mg/(Mg + Fe) cationic ratio] numerical modellingshows that decreasing Mg-number with increasing pyroxene proportion,characteristic of Ronda group C pyroxenites, can be accountedfor by a melt-consuming reaction resulting in the formationof mildly evolved, relatively low Mg-number melts (0·65)provided that the melt fraction during reaction and the time-integratedmelt/rock ratio are high enough (>0·1 and > 1,respectively) to balance Mg–Fe buffering by peridotiteminerals. This implies strong melt focusing caused by melt channellingin high-porosity domains resulting from compaction processesin a partial melted lithospheric domain below a solidus isothermrepresented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns ofgroup C whole-rocks and clinopyroxenes are convex-upward. Numericalmodeling of REE variations in clinopyroxene produced by a pyroxene-forming,melt-consuming reaction results in curved trajectories in the(Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized).Based on (Ce/Nd)N values, two transient, enriched domains betweenthe light REE (LREE)-depleted composition of the initial peridotiteand that of the infiltrated melt may be distinguished in thereaction column: (1) a lower domain characterized by convex-upwardREE patterns similar to those observed in Ronda group C pyroxenite–peridotite;(2) an upper domain characterized by melts with strongly LREE-enrichedcompositions. The latter are probably volatile-rich, small-volumemelt fractions residual after the refertilization reactionsthat generated group C pyroxenites, which migrated throughoutthe massif—including the unmelted lithospheric spinel-tectonitedomain. The Ronda mantle domains affected by pyroxenite- anddunite- or harzburgite-forming reactions (the ‘layeredgranular’ subdomain and ‘plagioclase-tectonite’domain) are on average more fertile than the residual, ‘coarsegranular’ subdomain at the recrystallization front. Thisindicates that refertilization traces the moving boundariesof receding cooling of a thinned and partially melted subcontinentallithosphere. This refertilization process may be widespreadduring transient thinning and melting of depleted subcontinentallithospheric mantle above upwelling asthenospheric mantle. KEY WORDS: subcontinental mantle; refertilization; pyroxenite; peridotite; websterite; melt–rock reaction; plagioclase; trace elements  相似文献   

3.
In the Speik Complex (Eastern Alps, Austria), highly melt-depleted,metamorphosed harzburgites with abundant pods and layers ofchromitite are interlayered with a suite of metamorphosed orthopyroxenites,clinopyroxenites and gabbros. Coarse-grained orthopyroxenitesoccur as centimetre- to metre-wide veinlets and pods, but alsoas intrusive plugs several tens of metres wide. Intimately associatedmetaclinopyroxenite and metagabbro are present as bodies upto several metres thick at a distinct stratigraphic level withinthe complex. In the ultramafic rocks, relict magmatic olivine,orthopyroxene, clinopyroxene and spinel have been overprintedby a metamorphic assemblage of forsterite, diopside, tremolite,anthophyllite, chlorite, serpentine, talc and Cr–Fe-richspinel. Hornblende, epidote, zoisite and chlorite dominate themetamorphic paragenesis in metagabbros, in addition to rarerelicts of clinopyroxene and two phases of Ca-rich garnet. Thepolymetamorphic evolution of the Speik Complex includes rarelypreserved pre-Variscan (400 Ma) eclogite-facies conditions,Variscan (330 Ma) amphibolite-facies conditions (600–700°C,>5 kbar) and Eoalpine (100 Ma) greenschist- to amphibolite-faciesconditions reaching 550°C and 7–10 kbar. Orthopyroxenitesare characterized by high concentrations of SiO2, MgO and Cr,and by U-shaped chondrite-normalized rare earth element (REE)patterns similar to those of their harzburgite hosts. The REEpatterns of the clinopyroxenites are flat to slightly enrichedin light REE. Metagabbro compositions are variable, but generallycharacterized by low SiO2 and high mg-numbers (61–78).Their REE patterns all have GdN/YbN > 1; some samples havelarge positive Eu anomalies implying the original presence ofcumulus plagioclase. In the orthopyroxenites, clinopyroxenitesand some peridotites, Pt, Pd and Re are distinctly enrichedcompared with Os, Ir and Ru, whereas most harzburgites haveunfractionated to slightly fractionated platinum-group element(PGE) patterns with respect to average upper mantle. The Re–Osisotope compositions of the pyroxenites define an errorchronat 550 ± 17 Ma and a supra-chondritic 187Os/188Os of0·179 ± 0·003. An isochron age of 554 ±37 Ma with Nd(i) +0·7 is indicated by the Sm–Ndisotope compositions of whole-rock pyroxenite and gabbro samples,whereas the harzburgites plot on an errorchron of 745 ±45 Ma and Nd(i) +6. The pyroxenites and gabbros probably representa cogenetic suite of magmatic dykes intruded into uppermost,highly depleted, suboceanic mantle below the crust–mantletransition zone in an oceanic basin close to the northwesternmargin of Gondwana. KEY WORDS: pyroxenite; metagabbro; geochemistry; Re–Os isotopes; Sm–Nd isotopes  相似文献   

4.
High-pressure Partial Melting of Mafic Lithologies in the Mantle   总被引：17，自引：2，他引：15  
We review experimental phase equilibria associated with partialmelting of mafic lithologies (pyroxenites) at high pressuresto reveal systematic relationships between bulk compositionsof pyroxenite and their melting relations. An important aspectof pyroxenite phase equilibria is the existence of the garnet–pyroxenethermal divide, defined by the enstatite–Ca-Tschermakspyroxene–diopside plane in CaO–MgO–Al2O3–SiO2projections. This divide appears at pressures above 2 GPa inthe natural system where garnet and pyroxenes are the principalresidual phases in pyroxenites. Bulk compositions that resideon either side of the divide have distinct phase assemblagesfrom subsolidus to liquidus and produce distinct types of partialmelt ranging from strongly nepheline-normative to quartz-normativecompositions. Solidus and liquidus locations are little affectedby the location of natural pyroxenite compositions relativeto the thermal divide and are instead controlled chiefly bybulk alkali contents and Mg-numbers. Changes in phase volumesof residual minerals also influence partial melt compositions.If olivine is absent during partial melting, expansion of thephase volume of garnet relative to clinopyroxene with increasingpressure produces liquids with high Ca/Al and low MgO comparedwith garnet peridotite-derived partial melts. KEY WORDS: experimental petrology; mantle heterogeneity; partial melting; phase equilibrium; pyroxenite  相似文献   

5.
The Early to Middle Jurassic Talkeetna Arc section exposed inthe Chugach Mountains of south–central Alaska is 5–18km wide and extends for over 150 km. This accreted island arcincludes exposures of upper mantle to volcanic upper crust.The section comprises six lithological units, in order of decreasingdepth: (1) residual upper mantle harzburgite (with lesser proportionsof dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lowercrustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanicrocks. The pyroxenites overlie residual mantle peridotite, withsome interfingering of the two along the contact. The basalgabbronorite overlies pyroxenite, again with some interfingeringof the two units along their contact. Lower crustal gabbronorite(10 km thick) includes abundant rocks with well-developed modallayering. The mid-crustal plutonic rocks include a heterogeneousassemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40%area), and concentrations of mafic dikes and chilled mafic inclusions.The volcanic rocks (7 km thick) range from basalt to rhyolite.Many of the evolved volcanic compositions are a result of fractionalcrystallization processes whose cumulate products are directlyobservable in the lower crustal gabbronorites. For example,Ti and Eu enrichments in lower crustal gabbronorites are mirroredby Ti and Eu depletions in evolved volcanic rocks. In addition,calculated parental liquids from ion microprobe analyses ofclinopyroxene in lower crustal gabbronorites indicate that theclinopyroxenes crystallized in equilibrium with liquids whosecompositions were the same as those of the volcanic rocks. Thecompositional variation of the main series of volcanic and chilledmafic rocks can be modeled through fractionation of observedphase compositions and phase proportions in lower crustal gabbronorite(i.e. cumulates). Primary, mantle-derived melts in the TalkeetnaArc underwent fractionation of pyroxenite at the base of thecrust. Our calculations suggest that more than 25 wt % of theprimary melts crystallized as pyroxenites at the base of thecrust. The discrepancy between the observed proportion of pyroxenites(less than 5% of the arc section) and the proportion requiredby crystal fractionation modeling (more than 25%) may be bestunderstood as the result of gravitational instability, withdense ultramafic cumulates, probably together with dense garnetgranulites, foundering into the underlying mantle during thetime when the Talkeetna Arc was magmatically active, or in theinitial phases of slow cooling (and sub-solidus garnet growth)immediately after the cessation of arc activity. KEY WORDS: island arc crust; layered gabbro; Alaska geology; island arc magmatism; lower crust  相似文献   

6.
Widespread bodies of garnet–spinel metaperidotites withpyroxenitic layers occur in the ultrahigh-pressure metamorphicKimi Complex. In this study we address the origin of such peridotite–pyroxeniteassociations in the context of polybaric melting regimes. Weconduct a detailed geochemical investigation of major and traceelement relations and compare them with a range of major elementmodelling scenarios. With increasing bulk-rock MgO content,the garnet–spinel metaperidotites exhibit decreasing CaO,Al2O3, TiO2, and Na2O along with increasing Ni and a graduallyincreasing Zr/Zr* anomaly, consistent with an origin as residuesafter variable degrees of melt extraction. The major elementmodelling further suggests a polybaric adiabatic decompressionmelting regime beginning at high to ultrahigh pressure, withan intermediate character between pure batch and fractionalmelting and a mean extent of melting of 9–11%. The pyroxenitesexhibit major element compositions that cannot be reproducedby experimental or calculated melts of peridotite. Moreover,the Kimi pyroxenites have highly variable Ni and Sc contentsand a wide range of Mg-number (0· 76–0·89), inconsistent with an origin as frozen melts or the productsof melt–peridotite interaction. However, both the majorelement systematics and the observed rare earth element patterns,with both convex and concave shapes, can be explained by anorigin as clinopyroxene-rich, high-pressure cumulates involvinggarnet and/or Cr-spinel. KEY WORDS: peridotite; pyroxenite; partial melting; UHP metamorphism; cumulate  相似文献   

7.
A geochemical and petrological study of Miocene to recent alkalibasalts, basanites, hawaiites, mugearites, trachytes, and phonoliteserupted within the Harrat Ash Shamah volcanic field was performedto reconstruct the magmatic evolution of southern Syria. Themajor element composition of the investigated lavas is mainlycontrolled by fractional crystallization of olivine, clinopyroxene,± Fe–Ti oxides and ± apatite; feldspar fractionationis restricted to the most evolved lavas. Na2O and SiO2 variationswithin uncontaminated, primitive lavas as well as variably fractionatedheavy rare earth element ratios suggest a formation by variabledegrees of partial melting of different garnet peridotite sourcestriggered, probably, by changes in mantle temperature. The isotopicrange as well as the variable trace element enrichment observedin the lavas imply derivation from both a volatile- and incompatibleelement-enriched asthenosphere and from a plume component. Inaddition, some lavas have been affected by crustal contamination.This effect is most prominent in evolved lavas older than 3·5Ma, which assimilated 30–40% of crustal material. In general,the periodicity of volcanism in conjunction with temporal changesin lava composition and melting regime suggest that the Syrianvolcanism was triggered by a pulsing mantle plume located underneathnorthwestern Arabia. KEY WORDS: 40Ar/39Ar ages; intraplate volcanism; mantle plume; partial melting; Syria  相似文献   

8.
The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km3 of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. 87Sr/86Sri(0·7039–0·7049), 143Nd/144Ndi (0·5127–0·5129)and 176Hf/177Hfi (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na2O, P2O5, K2O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting  相似文献   

9.
Neogene basanite lavas of Kozákov volcano, located alongthe Lusatian fault in the northeastern Czech Republic, containabundant anhydrous spinel lherzolite xenoliths that providean exceptionally continuous sampling of the upper two-thirdsof central European lithospheric mantle. The xenoliths yielda range of two-pyroxene equilibration temperatures from 680°Cto 1070°C, and are estimated to originate from depths of32–70 km, based on a tectonothermal model for basalticunderplating associated with Neogene rifting. The sub-Kozákovmantle is layered, consisting of an equigranular upper layer(32–43 km), a protogranular intermediate layer that containsspinel–pyroxene symplectites after garnet (43–67km), and an equigranular lower layer (67–70 km). Negativecorrelations of wt % TiO2, Al2O3, and CaO with MgO and clinopyroxenemode with Cr-number in the lherzolites record the effects ofpartial fusion and melt extraction; Y and Yb contents of clinopyroxeneand the Cr-number in spinel indicate 5 to 15% partial melting.Subsequent metasomatism of a depleted lherzolite protolith,probably by a silicate melt, produced enrichments in the largeion lithophile elements, light rare earth elements and highfield strength elements, and positive anomalies in primitivemantle normalized trace element patterns for P, Zr, and Hf.Although there are slight geochemical discontinuities at theboundaries between the three textural layers of mantle, theretends to be an overall decrease in the degree of depletion withdepth, accompanied by a decrease in the magnitude of metasomatism.Clinopyroxene separates from the intermediate protogranularlayer and the lower equigranular layer yield 143Nd/144Nd valuesof 0·51287–0·51307 (Nd = +4·6 to+8·4) and 87Sr/86Sr values of 0·70328–0·70339.Such values are intermediate with respect to the Nd–Srisotopic array defined by anhydrous spinel peridotite xenolithsfrom central Europe and are similar to those associated withthe present-day low-velocity anomaly in the upper mantle beneathEurope. The geochemical characteristics of the central Europeanlithospheric mantle reflect a complex evolution related to Devonianto Early Carboniferous plate convergence, accretion, and crustalthickening, Late Carboniferous to Permian extension and gravitationalcollapse, and Neogene rifting, lithospheric thinning, and magmatism. KEY WORDS: xenoliths; lithospheric mantle; REE–LILE–HFSE; Sr–Nd isotopes; Bohemian Massif  相似文献   

10.
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy  相似文献   

11.
The Southwest Indian Ridge (SWIR) at 9–16°E and 52–53°Sis characterized by ultra-slow, oblique spreading and containsone of the few documented occurrences of pyroxenite veins associatedwith abyssal peridotites. The origin of these uncommon lithologiesis still debated. We present a detailed study (including electronmicroprobe and laser ablation inductively coupled plasma massspectrometry) of spinel websterites collected during Cruise162, Leg 9, of the R.V. Knorr. Rare earth element patterns inclinopyroxenes (Cpx) lead us to discard a possible origin ofthe pyroxenites as residues from partial melting of garnet pyroxenites(i.e. relics of a layered mantle protolith). Their compositionand cumulate texture (when not obscured by mylonitization relatedto emplacement on the seafloor) are better interpreted in termsof fractional crystallization from a basaltic melt at relativelyhigh pressure. Evidence for a high pressure of crystallizationincludes the lack of plagioclase in the cumulate assemblageand the high Al2O3 contents of the pyroxenes: up to 5 wt % inorthopyroxene (Opx) and up to 7 wt % in Cpx. These values areamong the highest reported for pyroxenes in a mid-ocean ridgesetting. Sub-solidus breakdown of spinel to plagioclase (nowaltered) is observed in one sample, providing a rough estimateof the final equilibration pressure of these cumulates, around0· 6–0· 7 GPa (plagioclase–spineltransition for a bulk pyroxenite composition). The inferredpyroxenite parent melts were close to equilibrium with the associatedresidual peridotites; some samples have a slightly evolved compositionin terms of the Mg-number [Mg/(Mg + total Fe)]. These parentalmelts had major and trace element compositions consistent witha mid-ocean ridge basalt (MORB) affinity, although they werenot rigorously identical to MORB. Among other characteristics,these melts were relatively depleted in highly incompatibleelements. We propose that they correspond to the latest, shallowest,incremental melt fractions produced during fractional decompressionmelting of a normal MORB (N-MORB) mantle source. These meltsexperienced fractional crystallization as soon as they segregatedfrom the peridotite matrix, moved upward, and crossed the lithosphere–asthenosphereboundary (defined here as the base of the conductive lid). Asa consequence, these shallow melt fractions produced beneathmid-ocean ridges did not fully mix with melt fractions producedand extracted at greater depths. Our study provides concreteevidence for the actuality of pyroxene crystallization in meltchannels beneath mid-ocean ridges at relatively high pressures,a process frequently invoked to account for the ‘pyroxeneparadox’ in MORB petrogenesis. KEY WORDS: abyssal pyroxenites; cumulates; lithospheric mantle; melt migration; Southwest Indian Ridge  相似文献   

12.
The South Auckland Volcanic Field is a Pleistocene (1·59–0·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·0–4·6wt %), Nb (7–29 ppm), and (La/Yb)N (3·4–7·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·3–7·9wt %), Nb (32–102 ppm), and (La/Yb)N (12–47). GroupA has slightly higher 87Sr/86Sr, similar Nd, and lower 206Pb/204Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high 238U/204Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources  相似文献   

13.
The Beni Bousera ultramafic massif, Morocco, is composed ofperidotite with subordinate garnet pyroxenitc units which belongto two different families: (1) the Type I pyroxenites, whichare characterized by an Fe-enrichment trend; and (2) the TypeII pyroxenites, which are characterized by high but nearly constantMg/Fe ratios and highly variable concentrations of Ca and Al;the latter family includes corundum-bearing garnet pyroxeniteswhich resemble the peraluminous eclogites and grospydites describedas xenoliths in kimberlite diatremes. The Type II pyroxenites appear as layered sheets in the peridotite,and have granuloblastic metamorphic texture. They contain aprimary association of a coarse-grained assemblage (cpx + gt;cpx + gt + sp; cpx + gt + co), and a variety of secondary andtertiary associations includ ng clinopyrox-ene, orthopyroxene,olivine, spinel, corundum, sapphirine, plagioclase, and amphibole.The primary assemblage in the corundum-bearing pyroxenite ischaracterized by clinopyroxene rich in A12O3 (up to 20 wt%),and poor in Na2O (generally less than 2 wt.%). The clinopyroxenephase is therefore richer in the Ca-Ts molecule than in thejadeite molecule. On the other hand, the composition of theprimary and secondary clinopyroxene and garnet phases showsstrong variation across the pyroxenite sheets. These variationsexpress compositional variations of the rock system across thesheets. The cpx-gt associations indicate high temperatures (1200–1350?C) in the central parts of the sheets. The crystallizationpressure may have reached at least 20 kb in the corundum-bearingassemblages. The bulk-rock composition and the compatible element's behaviourin the Type II pyroxenite sheets suggest that the modal andcryptic layering mainly resulted from igneous fractionationprocesses. The REE patterns of corundum-bearing Type II pyroxeniteare characterized by low concentrations of HREE and by significantEu anomalies. These, together with the high bulk-rock Sr/Ndratios, suggest that plagioclase segregation may have playeda significant part in the rock genesis. These geochemical featuresare similar to those described, in the literature, in some low-pressure,plagioclase-bearing adcumulates (e.g., in the crustal sequenceof the Oman ophiolite). They are quite different from thoseobserved in the Type I pyroxenite sheets in the Beni Bouseramassif, whose geochemistry suggests that plagioclase playedno part in the fractionation process, whereas garnet probablyfractionated as an early igneous phase. The Type II pyroxenitesheets have a primary isotopic signature similar to MORB, basedon the composition of leached clinopyroxene. It is concluded that the Mg-rich Type II pyroxenite sheets resultedultimately from the fractionation of a basaltic melt at lowpressure, and from the accumulation of olivine, clinopyroxene,and plagioclase along dykes cross-cutting the surrounding peridotite.The close similarities with the geochemical features in theOman ophiolite lead us to suggest that these processes may havebeen operative in an oceanic crustal environment. The high-pressureand high-temperature crystallization of the ‘primary’cpx+gt + co assemblage was achieved deep in the mantle, aftersubduction and/or dragging down in convection currents of thisparticular piece of the (oceanic?) lithosphere. Further ascentmay have resulted in partial melting of peridotite and/or pyroxenite,and in the emplacement of the Type I pyroxenite sheets.  相似文献   

14.
The Yanshan Fold and Thrust Belt in eastern China has been intrudedby a series of alkalic igneous rocks, ranging in compositionfrom granite and rhyolite to syenite and trachyte. Laser ablationinductively coupled plasma mass spectrometry U–Pb analysesof zircon from three alkaline suites yield Early Cretaceousages of 130–117 Ma. Three groups of rocks have been identifiedbased on their mineralogical, geochemical and Sr–Nd–Hfisotope characteristics. The alkali granites and rhyolites areferroan and have low Al2O3, MgO, CaO, Sr, Ba and Eu concentrationsand high SiO2, total Fe2O3, K2O, Nb, Ga, Ta, Th and heavy rareearth element abundances and Ga/Al ratios. Geochemical dataand Sr-, Nd- and zircon Hf-isotopic compositions [(87Sr/86Sr)i= 0·7050–0·7164, Nd(t) = –8·4to –13·6 and Hf(t) = –5·7 to –16·8]indicate that they were probably generated by shallow dehydrationmelting of biotite- or hornblende-bearing granitoid crustalsource rocks and then mixed with contemporaneous magma froma mantle and/or lower crustal source. Ferroan syenites havedistinct geochemical features from those of the alkaline granitesand rhyolites, suggesting that they were produced by clinopyroxeneand plagioclase fractionation of melt derived from an enrichedmantle source, mixed with lower and upper crustal-derived magmas.The magnesian syenites and trachytes have Sr-, Nd- and zirconHf-isotopic compositions that are distinct from those of theferroan syenites. They were mainly derived from partial meltingof lower crustal materials, mixed with enriched mantle-derivedalkali basaltic magma. The emplacement of an alkali syenite–granite–rhyolitesuite, coeval with the formation of metamorphic core complexesand pull-apart basins in eastern China, indicates they formedin an extensional setting, possibly as a result of lithosphericthinning. KEY WORDS: alkaline rocks; zircon U–Pb dating; petrogenesis; crustal extension; Yanshan Fold and Thrust Belt; North China Craton  相似文献   

15.
The Serbian province of Tertiary ultrapotassic volcanism isrelated to a post-collisional tectonic regime that followedthe closure of the Tethyan Vardar Ocean by Late Cretaceous subductionbeneath the southern European continental margin. Rocks of thisprovince form two ultrapotassic groups; one with affinitiesto lamproites, which is concentrated mostly in the central partsof the Vardar ophiolitic suture zone, and the other with affinitiesto kamafugites, which crops out in volcanoes restricted to thewestern part of Serbia. The lamproitic group is characterizedby a wide range of 87Sr/86Sri (0·70735–0·71299)and 143Nd/144Ndi (0·51251–0·51216), whereasthe kamafugitic group is isotopically more homogeneous witha limited range of 87Sr/86Sri (0·70599–0·70674)and 143Nd/144Ndi (0·51263–0·51256). ThePb isotope compositions of both groups are very similar (206Pb/204Pb18·58–18·83, 207Pb/204Pb 15·62–15·70and 208Pb/204Pb 38·74–38·99), falling withinthe pelagic sediment field and resembling Mesozoic flysch sedimentsfrom the Vardar suture zone. The Sr and Nd isotopic signaturesof the primitive lamproitic rocks correlate with rare earthelement fractionation and enrichment of most high field strengthelements (HFSE), and can be explained by melting of a heterogeneousmantle source consisting of metasomatic veins with phlogopite,clinopyroxene and F-apatite that are out of isotopic equilibriumwith the peridotite wall-rock. Decompression melting, with varyingcontributions from depleted peridotite and ultramafic veinsto the final melt, accounts for consistent HFSE enrichment andisotopic variations in the lamproitic group. Conversely, themost primitive kamafugitic rocks show relatively uniform Srand Nd isotopic compositions and trace element patterns, andsmall but regular variations of HFSE, indicating variable degreesof partial melting of a relatively homogeneously metasomatizedmantle source. Geochemical modelling supports a role for phlogopite,apatite and Ti-oxide in the source of the kamafugitic rocks.The presence of two contrasting ultrapotassic suites in a restrictedgeographical area is attributable to the complex geodynamicsituation involving recent collision of a number of microcontinentswith contrasting histories and metasomatic imprints in theirmantle lithosphere. The geochemistry of the Serbian ultrapotassicrocks suggests that the enrichment events that modified thesource of both lamproitic and kamafugitic groups were relatedto Mesozoic subduction events. The postcollisional environmentof the northern Balkan region with many extensional episodesis consistent at regional and local levels with the occurrenceof ultrapotassic rocks, providing a straightforward relationshipbetween geodynamics and volcanism. KEY WORDS: kamafugite; lamproite; Mediterranean; Serbia; mantle metasomatism; veined mantle; petrogenesis  相似文献   

16.
Mineralogical, isotopic, geochemical and geochronological evidencedemonstrates that the Friningen body, a garnet peridotite bodycontaining garnet pyroxenite layers in the Seve Nappe Complex(SNC) of Northern Jämtland, Sweden, represents old, certainlyProterozoic and possibly Archean, lithosphere that became incorporatedinto the Caledonian tectonic edifice during crustal subductioninto the mantle at c. 450 Ma. Both garnet peridotite and pyroxenitecontain two (M1 and M2) generations of garnet-bearing assemblagesseparated by the formation of two-pyroxene, spinel symplectitearound the M1 garnet and the crystallization of low-Cr spinel1Cin the matrix. These textures suggest initial high-pressure(HP) crystallization of garnet peridotite and pyroxenite succeededby decompression into the spinel stability field, followed byrecompression into the garnet peridotite facies. Some pyroxenitelayers appear to be characterized solely by M2 assemblages withstretched garnet as large as several centimeters. Laser ablationmicroprobe–inductively coupled plasma mass spectrometryRe–Os analyses of single sulfide grains generally definemeaningless model ages suggesting more than one episode of Reand/or Os addition and/or loss to the body. Pentlandite grainsfrom a single polished slab of one garnet peridotite, however,define a linear array on an Re–Os isochron diagram that,if interpreted as an errorchron, suggests an Archean melt extractionevent that left behind the depleted dunite and harzburgite bodiesthat characterize the SNC. Refertilization of this mantle bymelts associated with the development of the pyroxenite layersis indicated by enriched clinopyroxene Sr–Nd isotope ratios,and by parallel large ion lithophile-enriched trace elementpatterns in clinopyroxene from pyroxenite and the immediatelyadjacent peridotite. Clinopyroxene and whole-rock model Sm–Ndages (TDM = 1·1–2·2 Ga) indicate that fertilizationtook place in Proterozoic times. Sm–Nd garnet2–clinopyroxene2–wholerock ± orthopyroxene2 mineral isochrons from three pyroxenitelayers define overlapping ages of 452·1 ± 7·5and 448 ± 13 Ma and 451 ± 43 Ma (2  相似文献   

17.
Alpine-type peridotites and associated pyroxenites are foundas lenses in the continental crust in many different orogens.The reconstruction of the pressure–temperature (P–T)evolution of these rocks is, however, difficult or even impossible.With geothermobarometry, usually one point on the overall P–Tpath can be obtained. To use the different mineral assemblagesobserved in ultramafic rocks as P–T indicators, quantitativeP–T phase diagrams are required. This study presents newcalculated phase diagrams for peridotitic and pyroxenitic rocksin the model systems CaO–MgO–Al2O3–SiO2–H2O(CMASH) and Na2O–CaO–MgO–Al2O3–SiO2–H2O(NCMASH), which include the respective solid solutions as continuousexchange vectors. These phase diagrams represent applicablepetrogenetic grids for peridotite and pyroxenite. On the basisof these general petrogenetic grids, phase diagrams for particularperidotite and pyroxenite bulk compositions are constructed.In an example of pyroxenite from the Shackleton Range, Antarctica,the different observed mineral assemblages are reflected bythe phase diagrams. For these rocks, a high-pressure metamorphicstage around 18 kbar and an anticlockwise P–T evolution,not recognized previously, can be inferred. KEY WORDS: Antarctic; high-pressure metamorphism; peridotite; phase diagrams; pyroxenite  相似文献   

18.
The <80 ka basalts–basanites of the Potrillo VolcanicField (PVF) form scattered scoria cones, lava flows and maarsadjacent to the New Mexico–Mexico border. MgO ranges upto 12·5%; lavas with MgO < 10·7% have fractionatedboth olivine and clinopyroxene. Cumulate fragments are commonin the lavas, as are subhedral megacrysts of aluminous clinopyroxene(with pleonaste inclusions) and kaersutitic amphibole. REE modellingindicates that these megacrysts could be in equilibrium withthe PVF melts at 1·6–1·7 GPa pressure. Thelavas fall into two geochemical groups: the Main Series (85%of lavas) have major- and trace-element abundances and ratiosclosely resembling those of worldwide ocean-island alkali basaltsand basanites (OIB); the Low-K Series (15%) differ principallyby having relatively low K2O and Rb contents. Otherwise, theyare chemically indistinguishable from the Main Series lavas.Sr- and Nd-isotopic ratios in the two series are identical andvary by scarcely more than analytical error, averaging 87Sr/86Sr= 0·70308 (SD = 0·00004) and 143Nd/144Nd = 0·512952(SD=0·000025). Such compositions would be expected ifboth series originated from the same mantle source, with Low-Kmelts generated when amphibole remained in the residuum. ThreePVF lavas have very low Os contents (<14 ppt) and appearto have become contaminated by crustal Os. One Main Series picritehas 209 ppt Os and has a Os value of +13·6, typical forOIB. This contrasts with published 187Os/188Os ratios for KilbourneHole peridotite mantle xenoliths, which give mostly negativeOs values and show that Proterozoic lithospheric mantle formsa thick Mechanical Boundary Layer (MBL) that extends to 70 kmdepth beneath the PVF area. The calculated mean primary magma,in equilibrium with Fo89, has Na2O and FeO contents that givea lherzolite decompression melting trajectory from 2·8GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modellingof REE abundances in Main Series Mg-rich lavas is successfulfor a model invoking decompression melting of convecting sub-lithosphericlherzolite mantle (Nd = 6·4; Tp 1400°C) between90 and 70 km. Nevertheless, such a one-stage model cannot accountfor the genesis of the Low-K Series because amphibole wouldnot be stable within convecting mantle at Tf 1400°C. Thesemagmas can only be accommodated by a three-stage model thatenvisages a Thermal Boundary Layer (TBL) freezing conductivelyonto the 70 km base of the Proterozoic MBL during the 20 Myrtectonomagmatic quiescence before PVF eruptions. As it grew,this was veined by hydrous small-fraction melts from below.The geologically recent arrival of hotter-than-ambient (Tp 1400°C) convecting mantle beneath the Potrillo area re-meltedthe TBL and caused the magmatism. KEY WORDS: western USA; picrites; Sr–Nd–Os isotopes; petrogenetic modelling; thermal boundary layer  相似文献   

19.
Experimental Melting of Carbonated Peridotite at 6-10 GPa   总被引：2，自引：0，他引：2  
Partial melting of magnesite-bearing peridotites was studiedat 6–10 GPa and 1300–1700°C. Experiments wereperformed in a multianvil apparatus using natural mineral mixesas starting material placed into olivine containers and sealedin Pt capsules. Partial melts originated within the peridotitelayer, migrated outside the olivine container and formed poolsof quenched melts along the wall of the Pt capsule. This allowedthe analysis of even small melt fractions. Iron loss was nota problem, because the platinum near the olivine container becamesaturated in Fe as a result of the reaction Fe2SiO4Ol = FeFe–Ptalloy + FeSiO3Opx + O2. This reaction led to a gradual increasein oxygen fugacity within the capsules as expressed, for example,in high Fe3+ in garnet. Carbonatitic to kimberlite-like meltswere obtained that coexist with olivine + orthopyroxene + garnet± clinopyroxene ± magnesite depending on P–Tconditions. Kinetic experiments and a comparison of the chemistryof phases occasionally grown within the melt pools with thosein the residual peridotite allowed us to conclude that the meltshad approached equilibrium with peridotite. Melts in equilibriumwith a magnesite-bearing garnet lherzolite are rich in CaO (20–25wt %) at all pressures and show rather low MgO and SiO2 contents(20 and 10 wt %, respectively). Melts in equilibrium with amagnesite-bearing garnet harzburgite are richer in SiO2 andMgO. The contents of these oxides increase with temperature,whereas the CaO content becomes lower. Melts from magnesite-freeexperiments are richer in SiO2, but remain silicocarbonatitic.Partitioning of trace elements between melt and garnet was studiedin several experiments at 6 and 10 GPa. The melts are very richin incompatible elements, including large ion lithophile elements(LILE), Nb, Ta and light rare earth elements. Relative to theresidual peridotite, the melts show no significant depletionin high field strength elements over LILE. We conclude fromthe major and trace element characteristics of our experimentalmelts that primitive kimberlites cannot be a direct productof single-stage melting of an asthenospheric mantle. They rathermust be derived from a previously depleted and re-enriched mantleperidotite. KEY WORDS: multianvil; carbonatite melt; peridotite; kimberlite; element partitioning  相似文献   

20.
Major and trace element, Sr–Nd–Pb isotope and mineralchemical data are presented for newly discovered ultrapotassiclavas in the Tangra Yumco–Xuruco graben in southern Tibet.The ultrapotassic lavas are characterized by high MgO, K2O andTiO2, low Al2O3 and Na2O contents, and also have high molarK2O/Al2O3, molar (K2O + Na2O)/Al2O3 and K2O/Na2O ratios. Theirhigh abundances of incompatible trace elements such as largeion lithophile elements (LILE) and light rare earth elements(LREE) reach the extreme levels typical of lamproites. The lamproitesshow highly radiogenic 87Sr/86Sr (0· 7166–0·7363) and unradiogenic 143Nd/144Nd (0· 511796–0·511962), low 206Pb/204Pb (18· 459–18· 931),and elevated radiogenic 207Pb/204Pb (15· 6732–15·841) and 208Pb/204Pb (39· 557–40· 058) ratios.On the basis of their geochemical and isotopic systematics,the lamproites in south Tibet have a distinct magma source thatcan be differentiated from the sources of potassic lavas inthe east Lhasa and Qiangtang blocks. Their high Nb/Ta ratios(17· 10–19· 84), extremely high Th/U ratios(5· 70–13· 74) and distinctive isotope compositionsare compatible with a veined mantle source consisting of partialmelts of subducted Tethyan oceanic sediments and sub-continentallithospheric depleted mantle. Identification of the lamproitesand the delineation of their mantle source provide new evidencerelevant for models of the uplift and extension of the Tibetanplateau following the Indo-Asia collision. Metasomatism by partialmelts from isotopically evolved, old sediment subducted on theyoung Tethyan slab is an alternative explanation for PrecambrianNd and Pb model ages. In this model, differences in isotopiccomposition along-strike are attributed to differences in thetype of sediment being subducted, thus obviating the need formultiple metasomatic events over hundreds of million years.The distribution of lamproites, restricted within a north–south-trendinggraben, indicates that the initiation of east–west extensionin south Tibet started at 25 Ma. KEY WORDS: lamproites; subducted oceanic sediment; Tibetan active continental collision belt  相似文献   

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